Low-complexity deblocking filter

ABSTRACT

A method of filtering to remove coding artifacts introduced at block edges in a block-based video coder, the method having the steps of: checking the content activity on every line of samples belonging to a boundary to be filtered and where content activity is based on a set of adaptively selected thresholds determined using Variable-Shift Table Indexing (VSTI); determining whether the filtering process will modify the sample values on that particular line based on said content activity; and selecting a filtering mode between at least two filtering modes to apply on a block boundary basis, implying that there would be no switching between the two primary modes on a line by line basis along a given block boundary. The two filtering modes include a default mode based on a non-recursive filter, and a strong filtering mode which features two strong filtering sub-modes and a new selection criterion that is one-sided with respect to the block boundary to determine which of the two strong filtering sub-modes to use. The two strong filtering sub-modes include a new 3-tap filter sub-mode and a 5-tap filter sub-mode that permits a more efficient implementation of the filter.

(This application is a continuation of U.S. patent application Ser. No.10/310,059 filed Dec. 5, 2002 which is a division of U.S. patentapplication Ser. No. 10/300,849 filed Nov. 21, 2002. All of theseapplication are incorporated herein by reference.)

FIELD OF THE INVENTION

The present invention relates to the field of video coding, moreparticularly it relates to a method of reducing blocking artifactsinherent in hybrid block-based video coding.

BACKGROUND OF THE INVENTION

Video compression is used in many current and emerging products. It hasfound applications in video-conferencing, video streaming, serialstorage media, high definition television (HDTV), and broadcasttelevision. These applications benefit from video compression in thefact that they may require less storage space for archived videoinformation, less bandwidth for the transmission of the videoinformation from one point to another, or a combination of both.

Over the years, several standards for video compression have emerged;such as the Telecommunication Standardization Sector of theInternational Telecommunication Union (ITU-T) recommended video-codingstandards: H.261, H.262, H.263 and the emerging H.264 standard and theInternational Standardization Organization and InternationalElectrotechnical Commission (ISO/IEC) recommended standards MPEG-1,MPEG-2 and MPEG-4. These standards allow interoperability betweensystems designed by different manufacturers.

Video is composed of a stream of individual pictures (or frames) made upof discrete areas known as picture elements or pixels. The pixels areorganised into lines for display on a CRT or the like. Each pixel isrepresented as a set of values corresponding to the intensity levels ofthe luminance and chrominance components of a particular area of thepicture. Compression is based mainly on the recognition that much of theinformation in one frame is present in the next frame and, therefore, byproviding a signal based on the changes from frame to frame a muchreduced bandwidth is required. For the purpose of efficient coding ofvideo, the pictures or frames can be partitioned into individual blocksof 16 by 16 luminance pixels called “macroblocks”. This practicesimplifies the processing which needs to be done at each stage of thealgorithm by an encoder or decoded. To encode a macroblock (orsub-macroblock partition) using motion-compensated prediction, anestimation is made of the amount of motion that is present in the blockrelative to the decoded pixel data in one or more reference frames,usually recently decoded frames, and the appropriate manner in which toconvey the information from which the current frame may bereconstructed. The residual signal, which is the difference between theoriginal pixel data for the macroblock and its prediction, is spatiallytransformed and the resulting transform coefficients are quantizedbefore being entropy coded. The basic processing blocks of an encoderare a motion estimator/compensator/predictor, a transform, a quantizerand an entropy coder. Due to the quantization of the transformedcoefficients of the residual signal, the reconstructed pixel values aregenerally not identical to those of the original frame. Since the codingis block-based, the errors that are introduced by the quantizationprocess tend to produce artifacts in the form of sharp transitions inimage intensity across transform block boundaries in the reconstructedframe. Such artifacts are referred to as “blocking artifacts”. Theappearance of blocking significantly affects the natural smoothness seenin video images and leads to a degradation of the overall video imagequality.

Blocking artifacts are inherent in hybrid block-based video coders,especially in low bit rate video applications. A number of solutionshave been presented to alleviate the degradation in visual quality dueto the presence of blocking artifacts. Two general approaches have beenproposed to deal with blocking artifacts. The first approach is based onusing a deblocking filter in the decoder only as a post-processingstage, and applying the deblocking filter on the decoded andreconstructed video frames before they are displayed. The purpose of thefilter is to modify the sample values around the block boundaries inorder to smooth unnatural sharp transitions that have been introduced bythe block-based coding process. Having a deblocking filter appliedoutside of the motion-compensation loop can be viewed as an optionalprocess for the decoder, placing no requirements on the video encoder.However, this scheme has a disadvantage in that the reference framesthat are used for generating predictions for the coding of subsequentframes will contain blocking artifacts. This can lead to reduced codingefficiency and degraded visual quality. The second approach to reducethe visibility of blocking artifacts is to apply a deblocking filterinside the motion-compensation loop. In this case, the reference framesthat are used for generating predictions for subsequent encoded framesrepresent filtered reconstructed frames, generally providing improvedpredictions and improved compression and visual quality. In order tocreate identical predictions at both the encoder and decoder, thedeblocking filter (sometimes referred to as a “loop filter” if it isinside the motion-compensation loop) must be applied in both the encoderand the decoder.

In order to reduce the appearance of blocking artifacts, a number ofvideo coding standards, including H.263 version 2, and most recently theemerging H.264 video coding standard specify the use of a deblockingfilter inside the motion-compensation loop. In particular, the H.264video coding standard fully specifies a deblocking filter that is to beused inside the motion-compensation loop in both the encoder anddecoder.

One of the known prior art methods is described in a document “WorkingDraft Number 2, Revision 2 (WD-2)” by the Joint Video Team (JVT) ofISO/IEC MPEG and ITU-T VCEG. In this prior art method, filtering occurson the edges of 4×4 blocks in both the luminance and chrominancecomponents of each reconstructed video frame. The filtering takes placeon one 16×16 macroblock at a time, with macroblocks processed inraster-scan order throughout the frame. Within each macroblock, verticaledges are filtered first from left to right, followed by filtering ofthe horizontal edges, from top to bottom. The filtering of samples forone line-based filtering operation occurs along the boundary separatingunfiltered samples p₀, p₁, p₂, and p₃ on one side of the boundary, andunfiltered samples q₀, q₁, q₂, and q₃ on the other side, as illustratedin FIG. 3 a. The block boundary lies between samples p₀ and q₀. In somecases p₁, p₂ may indicate samples that have been modified by filteringof a previous block edge. For each line-based filtering operation,unfiltered samples will be referred to with lower-case letters, andfiltered samples with upper-case letters. For each block boundarysegment (consisting of 4 rows or columns of samples), a “Boundarystrength” parameter, referred to as “Bs”, is computed before filtering.The calculation of Bs is based on parameters that are used in encodingthe bounding blocks of each segment. Each segment is assigned a Bs valuefrom zero to four, with a value of zero indicating that no filteringwill take place, and a value of 4 indicating that the strongestfiltering mode will be used.

The process for determining Bs is as follows. For each boundary, adetermination is made as to whether either one of the two blocks thatneighbour the boundary is intra-coded. If either block is intra-coded,then a further determination is made as to whether the block boundary isalso a macroblock boundary. If the block boundary is also a macroblockboundary, then Bs=4, else Bs=3.

Otherwise, if neither block is intra-coded then a further determinationis made as to whether either block contains non-zero transformcoefficients. If either block contains non-zero coefficients then Bs=2;otherwise if a prediction of the two blocks is formed using differentreference frames or a different number of frames and if a pair of motionvectors from the two blocks reference the same frame and eithercomponent of this pair has a difference of more than one sample, thenBs=1; else Bs=0, in which case no filtering is performed on thisboundary. The value of boundary strength, Bs, for a specific blockboundary is determined by the encoding characteristics of the two 4×4blocks along the boundary. Therefore, the control of the filteringprocess for each individual block boundary is well localized. The blockboundary is filtered only when it is necessary, based on whether thecoding modes used for the neighbouring blocks are likely to produce avisible blocking artifact.

The known filtering process starts with the step of filtering each 4×4block edge in a reconstructed macroblock. The filtering “Boundarystrength” parameter, Bs, is computed and assigned based on the codingparameters used for luma. Block boundaries of chroma blocks correspondto block boundaries of luma blocks, therefore, the corresponding Bs forluma is also used for chroma boundaries.

Filtering takes place in the order described above on all boundarysegments with non-zero value for Bs. The following describes the processthat takes place for each line-based filtering operation. TABLE 1QP_(av) dependent activity threshold parameters α and β QP_(av) 0 1 2 34 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 α 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 4 5 6 7 9 10 12 β 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 3 3 3 4 4 4 6 6 QP_(av) 28 29 30 31 32 33 34 3536 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 α 14 17 20 24 28 33 3946 55 65 76 90 106 126 148 175 207 245 255 255 255 255 255 255 β 7 7 8 89 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18

A content activity check is performed. If the check is passed, filteringcontinues; otherwise, the sample values are not modified on this line ofthe boundary segment. The activity check makes use of a pair of activitythreshold parameters, ALPHA (α) and BETA (β), whose particular valuesare selected from the above Table 1, based on the average quantizationparameter (QP_(av)) used in coding each boundary segment. It is notedthat QP_(av) represents the average value of the quantization parametervalues used in encoding the two blocks that neighbour the boundary, withrounding of the average by truncation of any fractional part.Accordingly, the content activity check is passed if|p ₀ −q ₀|<ALPHA(α) AND |p ₁ −p ₀|<BETA(β) AND |q ₁ −q ₀<BETA(β).

Further, if this first content activity check is passed, and Bs is notequal to 4, default mode filtering is performed. Otherwise, if the checkis passed and Bs is equal to 4, a second, stricter activity check isperformed. This activity check involves the evaluation of the condition1<|p ₀ −q ₀|<(QP _(av)>>2) AND |p ₂ −p ₀|<BETA(β) AND |q ₂ −q₀|<BETA(β).If this second condition is true on a particular line of samples, astrong mode filtering is used on this line of samples. Otherwise, adefault mode filtering is used on this line of samples. It should benoted the symbol “>>” is used to represent the operation of bit-wiseshifting to the right.

Among the disadvantages of the above described known method is that itpermits switching between two filtering modes with very differentcharacteristics at the level of each line of samples within a boundarysegment. This switching adds complexity to the filtering process and cansignificantly increase the worst-case critical path for processing onmany architectures.

Further disadvantages include the particular values in the tables offiltering parameters, ALPHA (α) and BETA (β), which are not optimized toproduce the best subjective viewing quality of reconstructed andfiltered video. Further, the characteristics of the deblocking filter interms of the threshold parameters used in the activity checks andequations used for generating filtered sample values are fixed in theknown method, providing the encoder with little or no flexibility tocontrol the properties of the deblocking filter. This hindersoptimization of the subjective quality of the decoded video fordifferent types of video content and displays.

In the default mode of the above identified filtering method, the valueΔ, which represents the change from the unfiltered values of p₀ and q₀to their respective filtered values is computed using:Δ=Clip(−C,C,(((q ₀ −p ₀)<<2+(p ₁ −q ₁)+4)>>3)),where C is determined as specified below and the function “Clip” isdefined as:Clip(a,b,c)=IF(c<a)THEN a ELSE IF(c>b) THEN b ELSE cFurther, the filtered values P₀ and Q₀ are computed whereP ₀=Clip(0,255,p ₀+Δ) and Q ₀=Clip(0,255,q ₀−Δ).

In order to compute the clipping value, C, that is used to determine Δ,and also determine whether the values of p₁ and q₁ will be modified onthis set of samples, two intermediate variables, a_(p) and a_(q) arecomputed, where:a _(p) =|p ₂ −p ₀| and a _(q) =|q ₂ −q ₀|.

If a_(p)<β for a luminance edge, a filtered sample P₁ is produced asspecified by:P ₁ =p ₁+Clip(−C0,C0, (p ₂ +P ₀−(p ₁<<1))>>1).

If a_(q)<β for a luminance edge, a filtered sample Q₁ is produced asspecified by Q₁=q₁+Clip(−C0,C0,(q₂+Q₀−(q₁<<1))>>1) where C0 is specifiedin Table 2 (see below), based on Bs and QP_(av) for the block boundary.For both luma and chroma, C is determined by setting it equal to C0 andthen incrementing it by one if a_(p)<β, and again by one if a_(q)<β.TABLE 2 Value of filter clipping parameter C0 as a function of QP_(av)and Bs QP_(av) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2122 23 24 25 Bs = 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1Bs = 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 Bs = 3 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 QP_(av) 26 27 28 29 30 3132 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Bs = 1 1 1 11 1 1 1 2 2 2 2 3 3 3 4 4 4 5 6 6 7 8 9 10 11 13 Bs = 2 1 1 1 1 1 2 2 22 3 3 3 4 4 5 5 6 7 8 8 10 11 12 13 15 17 Bs = 3 1 2 2 2 2 3 3 3 4 4 4 56 6 7 8 9 10 11 13 14 16 18 20 23 25

It is important to note that the computation of the filtered values P₁and Q₁ require as an input to the filtering equation the filtered valuesof P₀ and Q₀ from the current line of samples. This recursive filteringmethod presents a disadvantage as the values of P₀ and Q₀ must becomputed before the computation of P₁ and Q₁ can begin. This design canimpede parallel processing of the different samples and therebyincreases the critical path for the default mode filtering on mosthardware architectures.

An additional disadvantage in the default mode filtering process of theknown method is that the calculation of the clipping parameter, C, forchroma samples is unnecessarily complex. The chroma samples p₁ and q₁are never filtered in the default mode and, therefore, the computationof the variables a_(p) and a_(q) is only necessary to determine the Cparameter that is used to clip the value of Δ. These computations couldbe avoided by specifying a simpler method to compute C for chromafiltering.

For strong mode filtering in the known method, the following equationsare applied to calculate the filtered sample values:P ₀=(p ₂+2*p ₁+2*p ₀+2*q ₀ +q ₁+4)>>3,P ₁=(p ₃+2*p ₂+2*p ₁+2*p ₀ +q ₀+4)>>3,Q ₀=(p ₁+2*p ₀+2*q ₀+2*q ₁ +q ₂+4)>>3 andQ ₁=(p ₀+2*q ₀+2*q ₁+2*q ₂ +q ₃+4)>>3.For the luminance component only, p₂ and q₂ are also filtered asspecified by:P ₂=(2*p ₃+3*p ₂ +p ₁ +p ₀ +q ₂+4)>>3 andQ ₂=(2*q ₃+3*q ₂ +q ₁ +q ₀ +p ₀+4)>>3.

Filtering with this set of equations can lead to insufficient reductionin the visibility of blocking artifacts. It is therefore an object ofthe present invention to obviate or mitigate the above-mentioneddisadvantages.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda method of filtering samples to minimise coding artifacts introduced ata block boundary in a block-based video encoder, the method having thesteps of:

(a) calculating a pair of indices used to access a table of a pair ofcorresponding activity threshold values, the indices calculated using anaverage quantization parameter and an offset parameter;

(b) determining the activity threshold values based on the pair ofindicies;

(c) confirming whether the filtering process will modify the samplevalues on every line of samples for the block boundary by checking acontent activity for the every line of samples for the block boundary,the content activity based on the determined activity threshold values;and

(d) filtering the confirmed samples when a block on either side of theblock boundary was coded using inter prediction.

The determination of whether the filtering process will modify thesample values on each particular line is based on a content activitycheck which makes use of a set of adaptively selected thresholds whosevalues are determined using Variable-Shift Table Indexing (VSTI). Themethod is also operated on a system including tables for the variousactivity thresholds accessed through the calculated indicies.

In another aspect of the invention there is provided a method ofcontrolling filter properties to adjust the properties of said filter ata block boundary, the method having the steps of:

(a) computing an average quantization parameter value (QP_(av)) at theblock boundary;

(b) adding offset values Filter_Offset_A and Filter_Offset_B to theaverage quantization parameter value QP_(av) and clipping these valueswithin a given range to determine table indices Index_(A) and Index_(B);and

(c) accessing an ALPHA (α) table, a BETA (β) table, and a Clipping (C0)table using the indices computed based on the filter offsets and theaverage quantization parameter value such that:ALPHA=ALPHA_TABLE[Index_(A)]BETA=BETA_TABLE[Index_(B)]C0=CLIP_TABLE[Bs][Index_(A)]

In a still further aspect of the invention there is provided a method offiltering samples to minimise coding artifacts introduced at a blockboundary in a block-based video encoder, the method having the steps ofchecking content activity on every line of samples belonging to theboundary to be filtered and determining whether the filtering processwill modify the sample values on said line of samples based on contentactivity thresholds that are dependent on a quantization parameter anddetermined using a filter offset parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the inventionwill become more apparent in the following detailed description in whichreference is made to the appended drawings wherein:

FIG. 1 is a schematic representation of a data transmission system;

FIG. 2 is a schematic representation of hierarchy of levels of an H.264conformant bitstream;

FIG. 3 a is schematic representation of a macroblock and a block;

FIG. 3 b is a diagram showing relationship between unfiltered samplesand activity thresholds;

FIG. 4 is a block diagram of a hybrid block-based video decoderincluding a deblocking filter inside the motion compensation loop of thesystem of FIG. 1;

FIG. 5 is a flowchart of the operation of the deblocking filter processfor the decoder of FIG. 4;

FIG. 6 is the dependency graph for default mode filter for the decoderof FIG. 4; and

FIG. 7 is flowchart for the process of calculating the boundary strengthfor the decoder of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a video conferencing system 10 used as an exampleof a video transmission system has participants A and B that exchangevideo data 12 between monitors 13, formatted as a compressed bit stream15 over a network 14 (such as but not limited to the Internet). Eachparticipant A, B has a video processor 16 having an encoder 18 forencoding transmitted video data 12 and a decoder 20 for decoding thereceived bit stream 15. Each image frame 22 displayed on the monitors 13is made of a series of macroblocks 24, such as but not limited to ablock of 16×16 pixels, representing (for example) an object 26 whichmoves over a background 28 (for example a person giving a presentationwhile standing in front of a backdrop). Accordingly, the processors 16coordinate the display of successive frames 22 on the monitors 13, asthe video data 12 is communicated between the participants A, B, whichcan include applications such as video conferencing. It will beappreciated recognised that the system 10 may also involve the exchangeof video data 12 in the compressed bit stream 15 in either one directionor both and on peer-to-peer basis or broadcast.

The video data 12 is a temporal sequence of pictures, each referred toas a frame or field 22. Each picture 22 is organized as a matrix ofmacroblocks 24. Each macroblock 24 has a size of 16 by 16 pixels and themacroblocks 24 are stored from left to right and from top to bottom andgroups of macroblocks 24 are combined in a slice 32 (see FIG. 2).Generally, a slice 32 contains macroblocks 24 and each macroblock 24consists of blocks 25 (see FIG. 3). Generally, each macroblock 24 iscomposed of three images; one red (R), one green (G), and one blue (B).However, for compatibility with non-coloured media, the RGB model isrepresented as an equivalent YCbCr model, where Y is a luminance (luma)component, and Cb and Cr are chrominance (chroma) components, such thattypically Y=0.299R+0.587G+0.114B, Cb=B−Y, and Cr=R−Y. Therefore, eachframe 22 of the video data 12 is generically referred to as containingone luma image, one Cb chroma image, and one Cr chroma image. Standardformats have 8 bits per pixel to digitally represent each of the threecomponents, where Cb and Cr images are typically downsampled by 2 ineach dimension due to the sensitivity of human vision. Generally, eachblock 25 consists of four pixels for the luma components and one pixelfor each chroma component of the 4:2:0 color data. The blocks 25 areprocessed and compressed for transmission as the bit stream 15 over thenetwork 14 (see FIG. 1).

Generally, one of three fundamental coding modes can be selected foreach macroblock 24, with the choice of coding mode determining how theprediction of a macroblock 24 is formed. Intra-coded (I) macroblocks 24make use of intra-prediction, in which the prediction is formed usingonly the current picture. In predictive (P), or inter-coded, macroblocks24 the prediction of each sample is formed by referring to one block 25in the set of previously decoded and stored reference pictures 22. Inbi-predictive (B) macroblocks 24, predictions can be formed in this way,but can also be formed by computing a weighted average of two differentblocks 25 in the set of previously decoded reference pictures 22. Itwill be noted that some of the previously decoded pictures 22 aretypically temporally subsequent to the current picture in terms of theirintended display order when bi-predictive coding is used. Depending onthe mode of each slice 32, which is indicated in the slice header 27, P-and B-macroblocks 24 may not be permitted within certain slices 32.

Referring again to FIG. 2, the bitstream 15 is organized into ahierarchy of syntax levels, with the 3 main levels being a sequencelevel 17, a picture (or frame) level 19, and slice level 21. A conceptknow as “parameter sets” allows efficient transmission of infrequentlychanging data at the sequence 17 and picture level 19 in the H.264standard. A sequence parameter set 29 in the first level 17 includesvalues of parameters that will remain unchanged for an entire videosequence, or from one instantaneous decoder refresh (IDR) picture to thenext. (IDR pictures are used to provide points of random access into thebitstream). Examples of parameters in a sequence parameter set 29include frame dimensions and the maximum number of reference frames. Aunique ID number “N” identifies each sequence parameter set 29.

A picture parameter set 31 in the second level 21 includes values ofparameters that will remain unchanged within a coded representation of apicture (frame or field) 22. Examples of parameters in the pictureparameter set 31 include the entropy coding mode and a flag thatspecifies whether deblocking filter parameters will be transmitted inthe slice headers 27 of the picture 22 (see FIG. 1). Each pictureparameter set 31, labeled as “M”, refers to the unique ID of a validsequence parameter set 29, which selects the active sequence parametersthat are used when decoding coded pictures 22 that use the particularpicture parameter set 31. The unique ID number “M” identifies eachpicture parameter set 31.

A slice 32 in the bit stream 15 contains a picture data 35 representinga sub-set of the macroblocks 24 of the complete picture 22. Themacroblocks 24 in a slice 32 are ordered contiguously in raster scanorder. The coded slice 32 includes the slice header 27 and the slicedata 35 (coded macroblocks 24). The slice header 27 contains a codedrepresentation of data elements 35 that pertain to the decoding of theslice data that follow the slice header 27. One of these data elementscontains a reference to a valid picture parameter set 31, whichspecifies the picture parameter values (and indirectly the sequenceparameter values) to be used when decoding the slice data 35. Each sliceheader 27 within the same picture 22 must refer to the same pictureparameter set 31. Other data elements in the slice header 27 include theinitial quantization parameter for the first macroblock 24 in the slice32 and deblocking filter offset parameters 39 (as further explainedbelow), if the transmission of such offset parameters 39 is specified inthe active picture parameter set.

Thus, the filter offsets 39 are transmitted in the slice header 27, andtherefore the offsets 39 can be different for each slice 32 within thepicture 22. However, depending on the value of a flag in the pictureparameter set 31 (“filter_parameters_flag”), the transmission of theseoffsets 39 in the slice header 27 might be disabled. In the case thatoffsets 39 are not transmitted, a default value of zero is used for bothfilter offsets 39 for example. Further, each picture parameter set 31contains parameter values that pertain to the decoding of the pictures22 for which the particular parameter set 31 is active (i.e. selected inthe slice headers 27 of the picture 22). The parameter sets 31 alsocontain a reference to the sequence parameter sets 29, which are activefor decoding of the pictures 22. The choice of sequence parameter sets29 and picture parameter sets 31 can be chosen by the encoder 18 (seeFIG. 1), or set at the time of system 10 setup for sequential operationof the encoder 18, decoder 20 pair.

Referring further to FIG. 2, each of the pictures 22 can selectindividual picture parameter sets that specify the picture structure andthe picture coding type. For exemplary purposes only, FIG. 3 a containsthe macroblock 24 each consisting of a grouping of pixels, such as a16×16 luma block 25 with the two associated 8×8 chroma blocks 25.However, it is recognized that other sizes of blocks 24 could be used torepresent the frames 22, if desired. Each slice 32 of the frame 22 isencoded by the encoder 18 (see FIG. 1), independently from the otherslices 32 in the frame 22. Each of the slices 32 has the slice header 27that provides information, such as but not limited to the position ofthe respective slice 32 in the frame 22 as well as the initialquantization parameter; and the slice data which provides informationfor reconstructing the macroblocks 24 of a slice 32, such as but notlimited to the prediction modes and quantised coefficients for each ofthe respective macroblocks 24.

Referring to FIG. 4, the decoder 20 processes the received bit stream 15and then reconstructs the predicted frame 46, using a stored copy of thereference frame(s) 48, the transmitted motion vectors 23, and thedecompressed or reassembled prediction error 54 contained in the bitstream 15.

The bit stream 15 generated by the encoder 18 is processed by thedecoder 20 to produce the reconstructed video images 55. Referring toFIG. 4, the video decoder 20 is based on functional units or componentssimilar to those found in other hybrid block-based video decoders. Thefunctional units include a buffering unit 33 that receives thecompressed bitstream 15, an entropy decoder 34 which decodes thereceived bit stream 15 to produce syntax elements used in subsequentprocessing by the other decoder 20 components, a motion compensatedprediction 36 to produce the predicted frame, an inverse scanning andquantization unit 38, and inverse transform units 40 to reproduce thecoded prediction error 54. A reconstruction unit 42 adds the predictionerror 54 to the predicted pixels 57 to produce the reconstructed frame56, and a deblocking filter 44 that smoothes the edges of sub-blockswithin the reconstructed frame 56 to produce the filtered reconstructedframe 56. Each of the above mentioned components is discussed in moredetail in the following.

The incoming video bitstream 15 is stored in a buffer 33 at the input tothe decoder 20. The first stage in the decoding process includes theparsing and decoding of the entropy coded bitstream symbols that arestored in a buffer 46 to produce the syntax elements used by the otherdecoder 20 components.

The various syntax elements in the bitstream 15 are de-multiplexed foruse in different processes within the decoder 20. High-level syntaxelements include temporal information for each frame, frame coding typesand frame dimensions. The coding can be based primarily on macroblocks24 consisting of 16×16 luminance-pixel blocks 25 and 2 8×8 chrominancepixel blocks 25. On the macroblock 24 level, syntax elements include thecoding mode of the macroblock 24, information required for forming theprediction, such as motion vectors 23 and spatial prediction modes, andthe coded information of the residual (difference) blocks, such as thecoded block pattern (CBP) for each macroblock 24 and quantized transformcoefficients for each of the underlying blocks 24.

Depending on the coding mode of each macroblock 24, the predictedmacroblock 24 can be generated either temporally (inter prediction) orspatially (intra prediction). The prediction for an inter-codedmacroblock 24 is specified by the motion vectors 23 that are associatedwith that macroblock 24. The motion vectors 23 indicate the positionwithin the set of previously decoded frames from which each block ofpixels will be predicted. Each inter-coded macroblock 24 can bepartitioned in a number of different ways, using blocks of sevendifferent sizes, with luminance block sizes ranging from 16×16 pixels to4×4 pixels. Also, a special SKIP mode exists in which no motion vectordifference values 23 (or coded residual blocks) are transmitted and theprediction is taken from a location in the previous picture that ispredicted by the values of previously decoded motion vectors 23 ofmacroblocks 24 neighbouring the current macroblock 24. Thus, 0 to 16motion vectors 23 can be transmitted for each inter-coded macroblock 24.Additional predictive modes in which two different motion vectors 23correspond to each pixel and the sample values are computed using aweighted average are supported when bi-predictive macroblock types areemployed.

For each motion vector 23, a predicted block 25 must be computed by thedecoder 20 and then arranged with other blocks 24 to form the predictedmacroblock 24. Motion vectors 23 in H.264 are specified generally withquarter-pixel accuracy. Interpolation of the reference video frames isnecessary to determine the predicted macroblock 24 using sub-pixelaccurate motion vectors 23.

Multiple (previous for P-pictures) reference pictures 22 can also beused for motion-compensated prediction. Selection of a particularreference pictures 22 is made on an 8×8 sub-macroblock 24 basis, orlarger if a larger sub-macroblock partition size is used for generatingthe motion-compensated prediction. This feature can improve codingefficiency by providing a larger set of options from which to generate aprediction signal.

Two different modes are supported in intra prediction and coding ofmacroblocks 24. In the 4×4 Intra mode, each 4×4 block within amacroblock 24 can use a different prediction mode. In the 16×16 Intramode, a single prediction mode is used for the entire macroblock 24. Theprediction of intra-coded blocks 25 is always based on neighboring pixelvalues that have already been decoded and reconstructed.

The decoding of a residual (difference) macroblock 24 requires that anumber of transforms be performed on any blocks for which non-zerotransform coefficients were transmitted in the bitstream, along withassociated scanning and coefficient scaling operations. The transformsthat are required for each macroblock 24 are determined based on thecoding mode and the coded block pattern (CBP) of the macroblock 24. Thedecoding of a difference macroblock 24 is based primarily on thetransformation of 4×4 blocks 25 of both the luminance and chrominancepixels, although in some circumstances, a second-level transform must beperformed on the DC coefficients of a group of 4×4 blocks 25 formacroblocks 24 that are coded in the 16×16 Intra prediction mode.Additionally, a special 2×2 transform is applied to the 4 DCcoefficients of the chrominance residual blocks 25 of a macroblock 24.

The values of the quantized coefficients are parsed and decoded by theentropy decoder 34. These are put into their correct order based on therun values through the scanning process and then the levels, whichrepresent quantized transform coefficients, are scaled viamultiplication by a scaling factor. Finally, the necessary transform toreconstruct the coded residual signal for a block is performed on thescaled coefficients. The result of the transforms for each macroblock 24is added to the predicted macroblock 24 and stored in the reconstructedframe buffer 48.

In the final stage of the decoding process, the decoder 20 applies thenormative de-blocking filtering process, which reduces blockingartifacts that are introduced by the coding process. The filter 44 isapplied within the motion compensation loop, so both the encoder 18 anddecoder 20 must perform this filtering. The filtering is performed onthe 4×4 block edges of both luminance and chrominance components. Thetype of filter 44 used, the length of the filter and its strength aredependent on several coding parameters as well as picture content onboth sides of each edge. A stronger filtering mode is used if the edgelies on a macroblock boundary 49 where the block on one or both sides ofthe edge is coded using intra prediction. The length of the filtering isalso determined by the sample values over the edge, which determine theso-called “activity measures”. These activity measures determine whether0, 1, or 2 samples on either side of the edge are modified by thefilter.

Filtering is applied across the 4×4 block edges of both luminance andchrominance components. Looking at FIG. 3 a, the blocks 25 are separatedby boundaries or block edges 47, with unfiltered samples p₀, p₁, p₂ andp₃ on one side of the boundary 47 and unfiltered samples q₀, q₁, q₂ andq₃ on the other side, such that the boundary 47 lies between p₀ and q₀.In some cases p₁, p₂ may indicate samples that have been modified byfiltering of the previous block edge 47. The deblocking filter 44 (seeFIG. 4) is applied on the block boundaries 47 of each reconstructedframe 56, which helps to reduce the visibility of coding artifacts thatcan be introduced at those block boundaries 49. The filter 44 includes acontrol function that determines the appropriate filtering to apply. Thecontrol algorithm is illustrated by FIG. 5.

One of the parameters used to control the filtering process of all theblock boundaries 47 is the boundary strength, Bs. The procedure fordetermining the boundary strength, Bs, for the block boundary 47 betweentwo neighbouring blocks j and k is illustrated in FIG. 7. For each edge47, a determination is made as to whether either one of the two blocks jand k across the boundary 47 is intra-coded, in step 140. If eitherblock j or k is intra-coded then a further determination is made as towhether the block boundary 47 is also a macroblock boundary 49, in step152. If the block boundary 47 is also a macroblock boundary 49, thenBs=4 (step 154), else Bs=3 (step 156).

Otherwise, if neither block j or k is intra-coded then a furtherdetermination is made as to whether either block 25 contains non-zerocoefficients, in step 142. If either block 25 contains non-zerocoefficients then

Bs=2 (step 144), otherwise the following condition is applied:

R(j)≠R(k) or|V(j, x)−V(k, x)|≧1 pixel or |V(j, y)−V(k, y)|≧1 pixel,where R(j) is the reference picture 22 used for predicting block j, andV(j) is the motion vector 23 used for predicting block j, consisting ofx and y (horizontal and vertical) components. Therefore, if a predictionof the two blocks 25 is formed using different reference frames 22 or adifferent number of frames 22 or if a pair of motion vectors 23 from thetwo blocks 25 reference the same frame and either component of this pairhas a difference if more than one sample distance, then this conditionholds true and

Bs=1 (step 148);

else,

Bs=0 (step 150), in which case no filtering is performed.

The value of boundary strength, Bs, for a specific block boundary 47 isdetermined solely by characteristics of the two 4×4 blocks 24 across theboundary 47. Therefore, the control of the filtering process for eachindividual block boundary 47 is well localized. A block boundary 47 isfiltered only when it is necessary, so that unneeded computation andblurring can be effectively avoided.

The flowchart of FIG. 5 describes the filtering process starting withstep 100 for the purposes of filtering each 4×4 block edge 47 in areconstructed macroblock 24. The filtering “Boundary strength”parameter, Bs, is computed (102) and assigned for luma. Block boundaries47 of chroma blocks 25 always correspond to block boundaries 47 of lumablocks 25, therefore, the corresponding Bs for luma is also used forchroma boundaries 47. The boundary strength is based on the parametersthat are used in encoding the bounding blocks 25 of each segment (104).Each segment is assigned a Bs value from 0 to 4, with a value of zeroindicating that no filtering will take place (108), and a value of 4indicating that the strongest filtering mode will be used.

In step 110, the filtering process takes place for each line of sampleson the block boundary 47. The set of filtering operations that takeplace on one line of a block boundary is referred to as a line-basedfiltering operation. A content activity check at the boundary 47 betweenthe two blocks 25 is performed in step 112. The content activity measureis derived from the absolute value of the separation between samplevalues of p₀, p₁, q₀, q₁ on either side of the boundary 47. The activitycheck is based on two activity threshold parameters ALPHA (α) and BETA(β), whose particular values are selected based on the averagequantization parameter (QP_(av)) used in coding each boundary segment,as well as upon a pair of encoder 18 selected parameter values, referredto as Filter_Offset_A and Filter_Offset_B (referred to as 39 in FIG. 2).QP_(av) represents the average of the quantization parameter values usedin coding the two blocks 25 that neighbour the boundary 47, withrounding of the average by truncation of any fractional part. Thus, thecontent activity check is done by comparing difference in the unfilteredsample values p₀ and q₀ across the boundary 47 against the activitythreshold ALPHA (α), and the difference in the unfiltered sample valuesp₀ and p₁ on one side of the boundary 47 and unfiltered sample values q₀and q₁ on the other side of the boundary 47 against the activitythreshold and BETA (β), as shown in FIG. 3 b. A determination is made todiscover whether the activity on the line is above or below the activitythreshold. If the activity is above the threshold, the sample values arenot modified, otherwise filtering continues. The ALPHA (α) and BETA (β)values are considered as activity thresholds for the difference inmagnitude between sample values along the line of samples beingfiltered.

Referring to FIG. 3, the ALPHA (α) and BETA (β) parameters represent theactivity thresholds for the difference in the values of unfilteredsamples p₀, p₁, q₀, q₁ across the boundary 47. The content activitycheck is passed if:p ₀ −q ₀|<ALPHA(α) AND |p ₁ −p ₀|<BETA(β) AND |q ₁ −q ₀|<BETA(β)

The sets of samples p₀, p₁, q₀, q₁ across this edge 46 are only filteredif Bs is not equal to zero and the content activity check expressed inthe above condition is passed.

The values in the ALPHA (α)- and BETA (β)-tables used in the loop filterare optimal in terms of the resulting video visual quality and allowsome flexibility in the encoder 18 in terms of adjusting the filterparameters, such as the activity threshold parameters and maximum changein a sample value produced by the default filter, through control of theindexing of these tables. The strength of the deblocking filter 44refers to the magnitude of the change in sample intensities that iscaused by the filtering process. Generally, the strength of the filter44 varies with the coding mode, as well as the step-size used forquantization of the transform coefficients. Stronger filtering isapplied when the quantization step-size (and its corresponding“quantization parameter”, QP) are larger, since it is more likely thatlarge block artifacts are created when the quantization is coarse. Thus,flexibility in the properties of the loop filter 44 is provided byallowing the encoder 18 to select offsets 39 to the QP-based indicesused to address these tables. This adds flexibility to the filter 44,help making it more robust to different content, resolutions, displaytypes, and other encoder 18 decision characteristics.

The α- and β-tables of the loop filter 44 are QP-dependent thresholdsthat define the maximum amount of activity at an edge for which the edgewill still be filtered. The modified α-table of the preferred embodimentis based on the subjective evaluation of a number of sequences over theentire QP scale. In the preferred embodiment, the value of a doublesevery 6 QP as it is related directly to the quantization step size,which also doubles every 6 QP in the H.264 standard.

A determination is made to find the QP value below which a should bezero, such that the filter is no longer used for values of a which equalzero. Looking at Table 1, in sequences with smooth areas, blockingartifacts are clearly visible using QP=19, which is the largest QP forwhich a is equal to zero. Based on Table 3, filtering will take placefor QP values as low as 16, since blocking artifacts are still visiblein smooth areas. The β-table is also extended at the low QP end in orderto permit filtering at these lower QP values.

The content activity check (112) determines whether each sample line isto be filtered and uses the following specific values for a and β (114)as shown in Table 3 below, where the index used to access the tables isclipped to be within the range of valid QP values (0 to 51). TABLE 3Index_(A) (for α) or Index_(B) (for β) 0 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22 23 24 25 26 α 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 04 4 5 6 7 8 9 10 12 13 15 β 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 2 3 3 33 4 4 4 6 Index_(A) (for α) or Index_(B) (for β) 27 28 29 30 31 32 33 3435 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 α 17 20 22 25 28 3236 40 45 50 56 63 71 80 90 101 113 127 144 162 182 203 226 255 255 β 6 77 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18

Further, the particular values for a and β to be used on each blockboundary 47 do not only depend on QP, but additionally upon a pair ofparameter values, referred to as Filter_Offset_A and Filter_Offset_B,(referenced 39 in FIG. 2) that are transmitted in the higher-levelsyntax (sequence 17, picture 19, or preferably the slice level 21)within the video bitstream 15. These offsets 39 are added to the averageQP value between the blocks 24 in order to calculate the indices thatare used to access the tables of ALPHA (α) and BETA (β) values (114), aswell as the C0 table:Index_(A)=Clip(QP _(min) ,QP _(max) ,QP _(av) +Filter _(—) Offset _(—)A)Index_(B)=Clip(QP _(min) ,QP _(max) ,QP _(av) +Filter _(—) Offset _(—)B)

The variables QP_(min) and QP_(max) in the above equations represent theminimum and maximum permitted values, respectively, of the quantizationparameter QP, and for example can be such that but not limited to thevalues 0 and 51, respectively.

However, because the values Index_(B) and Index_(A) are limited to liein a predetermined interval, if any of the computed coefficients lieoutside the interval, those values are limited to the permitted range bythe “clip” function. The function “clip” is defined as:clip(a,b,c)=IF(c<a)THEN a ELSE IF(c>b)THEN b ELSE c

By default, Filter_Offset_A and Filter_Offset_B values 39 are bothassumed to have a value of zero. Further, within the default filtering,Index_(A) is also used to access the table of C0 values. Transmission ofthe Filter_Offset_A and Filter_Offset_B values 39 in the slice header 27(see FIG. 2) provides a means of adapting the properties of thedeblocking filter 44 in terms of the magnitude of the thresholds used inthe activity checks and the maximum change in sample values that can beproduced by the default filter 44. This flexibility helps to allow theencoder to achieve the optimal visual quality of the decoded andfiltered video. Typically, the semantic in the slice header 27slice_alpha_c0_offset_div2 specifies the offset 39 used in accessing theALPHA (α) and C0 deblocking filter tables for filtering operationscontrolled by the macroblocks 24 within the slice 32. The decoded valueof this parameter is in the range from +6 to −6, inclusive. From thisvalue, the offset 39 that shall be applied when addressing these tablesis computed as:Filter_Offset_(—) A=slice_alpha_(—) c0_offset_(—) div2<<1

If this value is not present in the slice header 27, then the value ofthis field shall be inferred to be zero.

Correspondingly, the semantic in the slice header 27 slice_beta_offsetdiv2 specifies the offset 39 used in accessing the BETA (β) deblockingfilter tables for filtering operations controlled by the macroblocks 24within the slice 32. The decoded value of this parameter is in the rangefrom +6 to −6, inclusive. From this value, the offset 39 that shall beapplied when addressing these tables is computed as:Filter_Offset_(—) B=slice_beta_offset_(—) div2<<1.

If this value 39 is not present in the slice header 27, then the valueof this field shall be inferred to be zero. The resulting Variable-ShiftTable Indexing (VSTI) method (using the offsets 39 to shift selection ofthe α-, β-, and clipping (C0) values) allows the decoder 20 to make useof the offset 39 that is specified on the individual slice 32 basis andthat will be added to the QP value used in indexing the α-, β-, andclipping (C0) tables. Thus,Alpha(α)=ALPHA_TABLE[Index_(A)]Beta(β)=BETA_TABLE[Index_(B)]C0=CLIP_TABLE[Bs][Index_(A)]

The offset 39 for indexing the clipping table is always the same as forthe α-table. In general, it is desired have α and the clipping valuesremain in sync, although a different offset 39 for β can be beneficial.The implementation of this method can be simplified even further byapplying the offset 39 to the base pointers that are used to access thetables. This way, the extra addition only occurs as often as the offset39 can be changed (on a per-slice basis), not every time the table isaccessed. Clipping of the index can be avoided by extending the tableswith the last value in the valid range of indices at each end of thetable.

A positive offset 39 results in more filtering by shifting a curve (ofα, β, or C0 values) to the left on a horizontal QP scale, while anegative offset 39 results in less filtering by shifting a curve to theright. The range of permitted offsets 39 is −12 to +12, in increments of2. This range is large enough to allow properties of the filter 44 tovary as widely, but is limited to limit additional memory requirementsand/or added complexity. This variable-shift method provides bothstronger and weaker filtering, and there is sufficient flexibility inthe range of values, with reasonable constraints on the amount ofvariation permitted in the filtering, while maintaining the doublingrate of 6 QP's for α, consistent with the quantization step size. Also,the clipping (C0) and α values remain in sync with each other.

The specific decision on the choice of offsets 39 is varied, anddependent upon the content, resolution, and opinion of the viewer.Generally, less filtering is needed for slowly changing, detailed areasand for high-resolution pictures 22, while more filtering (usingpositive offsets 39) is preferable for lower resolution pictures 22,especially with smooth areas and human faces. More filtering can providethe viewer with a feeling of smoother motion.

Referring again to FIG. 5, if the check is not passed in step 116, thesample values are not modified on this line (118), otherwise filteringcontinues. The selection of the filtering mode occurs at the blockboundary 47 level. More specifically, switching between the default-modefiltering and the strong-mode filtering does not occur on a line-to-linebasis, and default-mode filtering is not used for intra-coded macroblockboundaries 47. In step 120, a further determination is made as towhether the macroblocks 24 are intra-coded. If the macroblocks 24 arenot intracoded, then a default filter is applied in step 122, in whichthe edges 47 with Bs<4 are filtered by computing the filtered samples P₀and Q₀ based on the DELTA (Δ). The variable Δ represents the differencethe between the unfiltered samples p₀ and q₀ and their respectivefiltered samples, P₀ and Q₀, according to the following relation:Δ=Clip(−C,C,(((q ₀ −p ₀)<<2+(p ₁ −q ₁)+4)>>3))P ₀=Clip(0,255,p ₀+Δ)Q ₀=Clip(0,255,q ₀−Δ)

The two intermediate threshold variables a_(p) and a_(q) are used todetermine the clipping value for the default filtering of luminancesamples, as well as the choice of one of the two sub-modes of the strongmode filter, wherea _(p) =|p ₂ −p ₀| and a _(q) =|q ₂ −q ₀|.

Thus, for default-mode filtering (122), the calculations of filteredsamples P₁ and Q₁ are modified from the prior art to increase theparallelism of the filtering process. If a_(p)<β for a luma edge, afiltered P₁ sample generated as specified by:P ₁ =p ₁+Clip(−C0,C0,(p ₂+(p ₀ +q ₀)>>1−(p ₁<<1))>>1).

While if a_(q)<β for a luma edge, a filtered Q₁ sample generated asspecified by:Q ₁ =q ₁+Clip(−C0,C0,(q ₂+(p ₀ +q ₀)>>1−(q ₁<<1))>>1)where C0 is specified in Table 4. However, the adaptable parameterIndex_(A) is used to address the table, rather than QP_(av).

A dependency graph for the default mode filter with reduced criticalpath as shown in FIG. 6 shows that the complexity can be reducedsignificantly. By shortening the critical path, a reduced cost ofdefault filtering can be achieved and opportunities for parallelprocessing can be substantially increased, leading to reducedcomputational requirements. Also, from this figure, the complexity ofBs=4 filtering is potentially reduced by not permitting the filter 44 toswitch between default and strong filter modes on a line-by-line basisto help minimise branching stalls and control logic.

For luminance only, C, which represents the maximum change in the levelof intensity that the default filter can apply to the p₀ and q₀ samples,is determined by setting it equal to C0 and then incrementing it by oneif a_(p)<β, and again by one if a_(q)<β. In the default luma filtering,P₁ and Q₁ are filtered only if a_(p)<β and a_(q)<β, respectively,evaluate to true, while P₁ and Q₁ are never filtered for chroma.Therefore, for chrominance filtering, instead of doing thesecalculations, C can be defined with the basic relationship:C=C0+1

Thus, there is a no need to perform the calculations of a_(p) and a_(q)for chrominance and therefore no need to load the sample values p₂ andq₂. This can reduce the complexity of the default chroma filtering byapproximately 20%. There is no reduction in quality, either objective orsubjective, introduced by this simplification. TABLE 4 Index_(A) 0 1 2 34 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Bs = 1 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 Bs = 2 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 Bs = 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 01 1 1 1 1 1 1 1 1 Index_(A) 26 27 28 29 30 31 32 33 34 35 36 37 38 39 4041 42 43 44 45 46 47 48 49 50 51 Bs = 1 1 1 1 1 1 1 1 2 2 2 2 3 3 3 4 44 5 6 6 7 8 9 10 11 13 Bs = 2 1 1 1 1 1 2 2 2 2 3 3 3 4 4 5 5 6 7 8 8 1011 12 13 15 17 Bs = 3 1 2 2 2 2 3 3 3 4 4 4 5 6 6 7 8 9 10 11 13 14 1618 20 23 25

For strong mode filtering where Bs=4 and the initial activity thresholdcheck 112 has been passed, a further determination to check whether eachside of the boundary 47 meets an additional smoothness criteria isperformed in steps 124 and 126. The smoothness criteria for theleft/upper side of the boundary 47 is checked in step 124, while thesmoothness criteria for the right/lower side is checked in step 126.Thus, a choice between a 3-tap filter or a 5-tap filter for theleft/upper (P) or the right/lower (Q) side of the boundary 47 is made.If the smoothness criterion is not met on a particular side, a 3-tapfilter is used to filter only a single pixel on that side of theboundary 47.

Specifically, for strong-mode filtering:a _(p) =|p ₂ −p ₀|a _(q) =|q ₂ −q ₀|

Therefore, in step 124, for filtering of edges with Bs=4 if thefollowing condition holds truea_(p)<BETA(β) AND |p ₀ −q ₀|<((ALPHA(α)>>2)+2),then filtering of the left/upper side of the block edge is specified bythe equations (130)P ₀=(p ₂+2*p ₁+2*p ₀+2*q ₀ +q ₁+4)>>3P ₁=(p ₂ +p ₁ +p ₀ +q ₀+2)>>2

In the case of luminance filtering, then (130)P ₂=(2*p ₃+3*p ₂ +p ₁ +p ₀ +q ₀+4)>>3

Otherwise, if the above condition does not hold, then filter only P0using the 3-tap filter (128)P ₀=(2*p ₁ +p ₀ +q ₁+2)>>2

Identical but mirrored filters are applied to the right/lower side ofthe boundary 47, substituting q and Q for p and P, respectively, in theabove description (and vice-versa) (132, 134).

Therefore, if the following condition holds true (126):a_(p)<BETA(β) AND |p ₀ −q ₀|<((ALPHA(α)>>2)+2)filtering of the right/lower side of the block edge (134) is specifiedby the equationsQ ₀=(p ₁+2*p ₀+2*q ₀+2*q ₁ +q ₂+4)>>3Q ₁=(p ₀ +q ₀ +q ₁ +q ₂+2)>>2

In the case of luminance filtering, then (134)Q ₂=(2*q ₃+3*q ₂ +q ₁ +q ₀ +p ₀+4)>>3

Otherwise, if the above condition does not hold, then only P0 isfiltered with the 3-tap filter (132)Q ₀=(2*q ₁ +q ₀ +p ₁+2)>>2

The system 10 thus includes a set of equations for the strong modefiltering to generate samples P₁ and Q₁ that can provide a greaterreduction in the visibility of blocking artifacts than alternativeequations that were used in the prior known method. Typically, thefilters for samples P₁ and Q₁ consist of only 4 taps, as opposed to the5 taps used for the other filtered samples in this strongest filteringmode. However, this is referred to as a 5-tap filter, since 5 taps isthe maximum used for any sample. In addition to providing an improvedreduction in blocking artifacts, these equations for filtering P₁ and Q₁are simpler than those used in the prior art method, potentiallyreducing the complexity of the filter by a small amount.

The system 10 includes tables for ALPHA (α) and BETA (β) that canimprove the subjective quality of the filtered video and can alsospecify an efficient method to allow the encoder 18 to control thecharacteristics of the deblocking filter 44 by transmitting variableoffsets 39 that affect the QP-based indexing of these tables.

Although the invention has been described with reference to certainspecific embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the spirit and scope ofthe invention as outlined in the claims appended hereto.

1. A method of line-based (one-dimensional) spatial filtering ofreconstructed samples to minimise coding artifacts introduced at a blockboundary in a block-based video encoder and/or decoder the method havingthe steps of: (a) calculating a pair of indices used to access a tableof a pair of corresponding activity threshold values, the indicescalculated using an average quantization parameter and an offsetparameter; (b) determining the activity threshold values based on thepair of indicies; (c) confirming whether the filtering process willmodify the sample values on every line of samples for the block boundaryby checking a content activity for the every line of samples for saidblock boundary, the content activity based on the determined activitythreshold values; and (d) filtering the confirmed samples when thefiltering strength, Bs, is not equal to zero.
 2. The method of claim 1,wherein a pair of the activity threshold values are ALPHA (α) and BETA(β), and the average quantization parameter, QP_(av), is the average ofthe two quantization values used in coding the two blocks that neighbourthe block boundary.
 3. The method of claim 2, wherein the values of theactivity threshold values, ALPHA (α) and BETA (β), for each possiblevalues of the indices Index_(A) and Index_(B) are Index_(A) (for α) orIndex_(B) (for β) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2021 22 23 24 25 26 α 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 4 5 6 7 8 9 10 1213 15 β 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 2 3 3 3 3 4 4 4 6 Index_(A)(for α) or Index_(B) (for β) 27 28 29 30 31 32 33 34 35 36 37 38 39 4041 42 43 44 45 46 47 48 49 50 51 α 17 20 22 25 28 32 36 40 45 50 56 6371 80 90 101 113 127 144 162 182 203 226 255 255 β 6 7 7 8 8 9 9 10 1011 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18


4. The method of claim 1, wherein the following equations are used tocompute the filtered sample values at a second position from the blockboundary on either side, denoted P₁ and Q₁, for a luminance component:P ₁ =p ₁+Clip(−C0,C0,(p ₂+(p ₀ +q ₀)>>1−(p ₁<<1))>>1)Q ₁ =q ₁+Clip(−C0,C0,(q ₂+(p ₀ +q ₀)>1−(q ₁<<1))>>1)
 5. The method ofclaim 4, wherein the maximum change in the value of the samples thatneighbour the block boundary, p₀ and q₀, due to the filtering processfor chrominance components is computed asC=C0+1.
 6. A method of filtering according to claim 2, wherein twooffset parameter values are added to the average QP value, QP_(av), inorder to compute the indices used to address the tables of ALPHA (α) andBETA (β) activity threshold values, such thatIndex_(A)=Clip(QP _(min) ,QP _(max) ,QP _(av)+Filter_Offset_(—) A)Index_(B)=Clip(QP _(min) ,QP _(max) ,QP _(av)+Filter_Offset_(—) B)
 7. Amethod of filtering according to claim 4, wherein the value of the C0parameter used for clipping the change in the sample values is computedusing a value of Index_(A) of the indicies to address the table of C0values.
 8. (canceled)
 9. A method of line-based (one-dimensional)spatial filtering of reconstructed samples to minimise coding artifactsintroduced at a block boundary in a block-based video encoder and/ordecoder, the method having the steps of checking content activity onevery line of samples belonging to the boundary to be filtered anddetermining whether the filtering process will modify the sample valueson said line of samples based on content activity thresholds that aredependent on a quantization parameter and determined using a filteroffset parameter.